Rubble-Pile Asteroids Prove Surprisingly Hard to Destroy, Study Finds

In 2010, a JAXA spacecraft named Hayabusa came home from a peanut-shaped asteroid called Itokawa, 535 meters long, carrying more than a thousand particles of rock. Each was smaller than a grain of sand. They were the first samples ever returned from an asteroid, and more than a decade later they have given researchers an answer nobody expected.

The finding comes from a study published in the Proceedings of the National Academy of Sciences, written by Fred Jourdan and Nick Timms of Curtin University, who are also the authors of the account this piece draws on. They examined several of the Itokawa dust grains with two techniques. The first fires an electron beam at a particle and detects the electrons scattered back, which reveals whether the rock was ever shocked by a meteor impact. The second, argon-argon dating, uses a laser to measure how much radioactive decay has occurred in a crystal, and so gives the age of that impact.

The result: the collision that destroyed Itokawa's parent asteroid, and in doing so formed Itokawa, happened more than 4.2 billion years ago. That is nearly as old as the Solar System. Early analysis by the JAXA team had already shown that Itokawa formed after the complete destruction of a parent body at least 20 kilometers across. The new work shows how long the fragments have endured since. The authors call the age totally unexpected, since it means Itokawa has survived almost an order of magnitude longer than solid asteroids are predicted to.

That comparison matters because asteroids come in two main types, mostly concentrated in the asteroid belt. Monoliths are single solid chunks of rock, the kind most people picture. A monolith about a kilometer across is predicted to last only a few hundred million years in the belt, a short span against the age of the Solar System. Rubble pile asteroids are different: they are assembled entirely from fragments thrown off when earlier monoliths were wholly or partly destroyed. Until now, their durability was poorly understood.

The explanation is porosity. Itokawa is around 40 percent void, so almost half of it is empty space. When collisions strike, they crush the gaps between the rocks rather than shattering the rocks themselves. The authors liken it to a giant space cushion. The finding also implies rubble piles are more abundant in the belt than previously thought, because once formed they are very hard to destroy.

Other missions have found the same structure. Hayabusa's pictures of Itokawa first demonstrated that rubble pile asteroids exist. JAXA's visit to Ryugu, which is classified as potentially hazardous, and NASA's visit to Bennu showed more of them. In September 2022, NASA's DART mission struck Dimorphos to test whether a small spacecraft could deflect an asteroid, and it succeeded. Close-up images showed Dimorphos is a rubble pile too. These missions found such bodies to be porous and low in density. Because they are fragments of larger asteroids, they are also relatively small and hard to spot from Earth, which the authors say makes them a major threat to understand.

That is where the planetary defense question arrives. DART nudged its target's orbit, but the authors note that the transfer of kinetic energy between a small spacecraft and a rubble pile is very small, so these bodies naturally resist falling apart. For an imminent, unforeseen threat, they suggest a more aggressive approach might be needed, such as the shockwave of a nuclear blast in space, which could transfer far more energy to a cushioned asteroid and push it away.

Whether anyone should actually test a nuclear shockwave approach, the authors write, is an entirely different question, and one the study leaves open.